There have been constant improvements in Intelligence-Surveillance-Reconnaissance (ISR) detector technology, for example, increasing the wavelength detection range, decreasing detector footprint and reducing pixel size, all of which enable the system size and weight to be reduced. These improvements created an increased demand for ISR broad-multi-band optical systems; specifically including a high definition visible band (VIS, 0.34 μm to ˜0.75 μm) as well as the near infrared (NIR, 0.75 μm to 1.4 μm), the short wavelength infrared (SWIR, 1.4 μm to 3 μm), the mid-wavelength infrared (MWIR, 3 μm to 8 μm), and long wavelength infrared (LWIR, 8 μm to 15 μm) bands. A key component of such systems is reflective optics that have a silver coating thereon which enables the systems to achieve this spectral performance. However, historically the silver coatings have been a source of system failures due to the propensity of the silver coating to “break-down” or “corrode” over time. It may not be possible to completely protect future systems from some of the harsh environments in which they will operate, particularly environments that are hot, humid and contain salt. It is thus desirable to have a system having highly durable broad band silver coated optics.
There are several test procedures that are used to evaluate the durability performance of thin film coated optical components. Examples include military specification documents such as MIL-C-48497, MIL-F-48616 and MIL-PRF-13830B, which include tests that involve exposure to humidity, salt fog, salt solutions, temperature cycling, abrasion, and other test procedures others. The harshest of these tests is the “24-hour salt fog” test. While at the present time there are two groups that claim highly reflective mirror stacks, made by specific processes, that can pass the 24-hour salt fog test (Quantum Coating Inc. using a Denton Vacuum process (not described) and Lawrence Livermore Laboratories using a process described in U.S. Pat. No. 7,838,134), neither of these stacks meet the entire ISR spectral requirements, specifically they do not meet them for the LWIR range. The Quantum/Denton silver coating is called “X-1 Silver” by both parties; and the published information shows performance in the 0.4 μm to 0.7 μm range (2000 Society of Vacuum, Coaters 505/856-7188; 43rd Annual Technical Conference Proceedings (2000) ISSN 0737-5921), with no information for longer wavelengths into the infrared out to the LWIR range being given for this coating. This article also suggests the use of ion beam assisted deposition and substrate heating might further improve performance. However, substrate heating is not desirable when using some metallic substrates, for example 6061-Al substrates, because if the temperature is too high the mechanical strength and corrosion resistance of the substrate is decreased. Consequently, it is preferred that the substrate temperature be below the heat treating (˜415° C.) and stress relief (˜350° C.) temperatures of the 6061-Al substrates. Lawrence Livermore Laboratories, U.S. Pat. No. 7,838,134, claims the use of nitrides for the silver adhesion-interface layers while using a Si3N4 protective cap layer. The patent states that the “silver mirror is characterized by high reflectance in a broad spectral range of 300 nm in the UV to the far infrared (˜10000 nm)”, this range being 0.3 μm to 10 μm.
However, despite the advances made in the art, further extension of the silver mirror's reflectance properties, including the wavelength range into the LWIR and mirror durability, is desirable.